Editing IPCC:AR6/WGII/Chapter-7 (section)

==== 7.3.1.6 Projected Impacts on Pollution- and Aeroallergens-Related Health Outcomes ====

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''Global air pollution-related mortality attributable directly to climate change—the human health climate penalty associated with climate-induced changes in air quality—is'' likely ''to increase and partially counteract any decreases in air pollution-related mortality achieved through ambitious emission reduction scenarios or stabilisation of global temperature change at 2°C'' ( ''medium confidence'' ) ''.'' Demographic trends in aging and more vulnerable population are ''likely'' to be important determinants of future air quality—a human health climate penalty ''(high confidence)'' .

Poor air quality contributes to a range of NCDs, including cardiovascular, respiratory and neurological, commonly resulting in hospitalisation or death. This section considers the possible risks for health of future climate-related changes in ozone and PM. The climate penalty, the degree to which global warming could affect future air quality, is better understood for ozone than for PM ( [[#von%20Schneidemesser--2020|von Schneidemesser et al., 2020]] ). This is because increases in air temperature enhance ozone formation via associated photochemical processes ( [[#Archibald--2020|Archibald et al., 2020]] ; [[#Fu--2019|Fu and Tian, 2019]] ). The association between climate and PM is complex and moderated by a diverse range of PM components as well as formation and removal mechanisms ( [[#von%20Schneidemesser--2020|von Schneidemesser et al., 2020]] ), added to which is uncertainty about future climate-related PM sources such as wildfires ( [[#Ford--2018|Ford et al., 2018]] ) and changes in aridity ( [[#Achakulwisut--2019|Achakulwisut et al., 2019]] ). As noted in AR6 WGI [[IPCC:Wg2:Chapter:Chapter-6|Chapter 6]] (Naik et al 2021), future air quality will largely depend on precursor emissions, with climate change projected to have mixed effects. Because of the uncertainty in how natural processes will respond, there is ''low confidence'' in the projections of surface ozone and PM under climate change ( [[#Naik--2021|Naik et al., 2021]] ). This has implications on the levels of confidence in the projections of the health climate penalty associated with climate-induced changes in air quality ( [[#Orru--2017|Orru et al., 2017]] ; [[#Orru--2019|Orru et al., 2019]] ; [[#Silva--2017|Silva et al., 2017]] ).

There is a rich literature on global and regional level projections of air quality-related health effects arising from changes in emissions. Comparatively few studies assess how changes in air pollution directly attributable to climate change are ''likely'' to affect future mortality levels. Projections indicate that emission reduction scenarios consistent with stabilisation of global temperature change at 2°C or below would yield substantial co-benefits for air quality-related health outcomes ( [[#Chowdhury--2018b|Chowdhury et al., 2018b]] ; [[#von%20Schneidemesser--2020|von Schneidemesser et al., 2020]] ; [[#Silva--2016c|Silva et al., 2016c]] ; [[#Markandya--2018|Markandya et al., 2018]] ; [[#Orru--2019|Orru et al., 2019]] ; [[#Shindell--2018|Shindell et al., 2018]] ) ( ''high confidence'' ). For example, by 2030, compared to 2000, it was estimated that globally and annually 289,000 PM2.5-related premature deaths could have been avoided under RCP4.5 compared to 17,200 PM2.5-related excess premature deaths under RCP8.5 ( [[#Silva--2016c|Silva et al., 2016c]] ). Further, and notwithstanding estimated reductions in global PM2.5 levels and an associated increase in the number of avoidable deaths, the benefits of following a low emissions pathway are expected to be apparent by 2100, with avoidable deaths estimated at 2.39 million deaths yr –1 under RCP4.5. This contrasts with the 1.31 million deaths estimated under RCP8.5. A few projections of the health-related climate penalty indicate a possible increase in ozone and PM2.5-associated mortality under RCP8.5 ( [[#Doherty--2017|Doherty et al., 2017]] ; [[#Orru--2019|Orru et al., 2019]] ; [[#Silva--2017|Silva et al., 2017]] ).

At the global level for PM2.5, annual premature deaths due to climate change were projected to be 55,600 (−34,300 to 164,000) and 215,000 (−76,100 to 595,000) in 2030 and 2100, respectively, countering by 16% the projected decline in PM2.5-related mortality between 2000 and 2100 without climate change ( [[#Silva--2017|Silva et al., 2017]] ). Similarly for ozone, the number of annual premature ozone-related deaths due to climate change was projected to be 3,340 in 2030 and 43,600 in 2050, with climate change accounting for 1.2% (14%) of the annual premature deaths in 2030 (2100) ( [[#Silva--2017|Silva et al., 2017]] ). These global level projections average over considerable geographical variations ( [[#Silva--2017|Silva et al., 2017]] ). Projections of the climate change effect on ozone mortality in 2100 were greatest for East Asia (41 deaths yr –1 per million people), India (8 deaths yr –1 per million people) and North America (13 deaths yr –1 per million people). For PM2.5, mortality was projected to increase across all regions except Africa (−25,200 deaths yr –1 per million people) by 2100, with estimated increases greatest for India (40 deaths yr –1 per million people), the Middle East (45 deaths yr –1 per million people), East Asia (43 deaths yr –1 per million people) and the Former Soviet Union (57 deaths yr –1 per million people). Overall, higher ozone-related health burdens were projected to occur in highly populated regions, and greater PM2.5 health burdens were projected in high PM emission regions ( [[#Doherty--2017|Doherty et al., 2017]] ).

For central and southern Europe, climate change alone could result in an 11% increase in ozone-associated mortality by 2050. However, projected declines in ozone precursor emissions could reduce the EU-wide climate change effect on ozone-related mortality by up to 30%; the reduction was projected to be approximately 24% if aging and an increasingly susceptible population were accounted for in projections to 2050 ( [[#Orru--2019|Orru et al., 2019]] ). For the USA in 2069, the impact of climate change alone on annual PM2.5- and ozone-related deaths was estimated to be 13,000 and 3,000 deaths, respectively, with heat-driven adaptation of air conditioning accounting for 645 and 315 of the PM2.5- and ozone-related annual excess deaths, respectively ( [[#Abel--2018|Abel et al., 2018]] ). An aging population is a determinant of future air quality-related mortality levels. An aging population along with an increase in the number of vulnerable people may work to offset the decrease in deaths associated with a low emission pathway (RCP4.5) and possibly dominate the net increase in deaths under a business as usual pathway (RCP8.5) ( [[#Chen--2020|Chen et al., 2020]] ; [[#Doherty--2017|Doherty et al., 2017]] ; [[#Hong--2019|Hong et al., 2019]] ; [[#Schucht--2015|Schucht et al., 2015]] ).

Complementing the longer-term changes in air quality arising from climate change are those associated with air pollution sensitive short-term meteorological events, such as heatwaves. Studies of individual heat events ( [[#Garrido-Perez--2019|Garrido-Perez et al., 2019]] ; [[#Johansson--2020|Johansson et al., 2020]] ; [[#Kalisa--2018|Kalisa et al., 2018]] ; [[#Pu--2017|Pu et al., 2017]] ; [[#Pyrgou--2018|Pyrgou et al., 2018]] ; [[#Schnell--2017|Schnell and Prather, 2017]] ; [[#Varotsos--2019|Varotsos et al., 2019]] ) and systematic reviews ( [[#Anenberg--2020|Anenberg et al., 2020]] ) provide evidence for synergistic effects of heat and air pollution. However, the health consequences of a possible additive effect of air pollutants during heatwave events were heterogeneous, varying by location and moderated by socioeconomic factors at the intra-urban scale ( [[#Analitis--2014|Analitis et al., 2014]] ; [[#Fenech--2019|Fenech et al., 2019]] ; [[#Krug--2020|Krug et al., 2020]] ; [[#Pascal--2021|Pascal et al., 2021]] ; [[#Schwarz--2021|Schwarz et al., 2021]] ; [[#Scortichini--2018|Scortichini et al., 2018]] ). This, combined with the challenges associated with projecting future concentrations of health-relevant pollutants during heatwave events ( [[#Jahn--2021|Jahn and Hertig, 2021]] ; [[#Meehl--2018|Meehl et al., 2018]] ), makes it difficult to say with any certainty that synergistic effects of heat and poor air quality will result in a heatwave–air pollution health penalty under climate change.

''The burden of disease associated with aeroallergens is anticipated to grow due to climate change'' ( ''high confidence'' ) ''.'' The incidence of pollen allergy and associated allergic disease increases with pollen exposure, and the timing of the pollen season and pollen concentrations are expected to change under climate change ( [[#Beggs--2021|Beggs, 2021]] ; [[#Ziska--2019|Ziska et al., 2019]] ; [[#Ziska--2020|Ziska, 2020]] ). The overall length of the pollen season and total seasonal pollen counts/concentrations for allergenic species such as birch ( ''Betula'' ) and ragweed ( ''Ambrosia'' ) are expected to increase as a result of CO 2 fertilisation and warming, leading to greater sensitisation ( [[#Hamaoui-Laguel--2015|Hamaoui-Laguel et al., 2015]] ; [[#Lake--2017|Lake et al., 2017]] ; [[#Zhang--2013|Zhang et al., 2013]] ). Changes in pollen levels for several species of trees and grasses are projected to increase annual emergency department visits in the USA by between 8% for RCP4.5 and 14% for RCP8.5 by the year 2090 ( [[#Neumann--2019|Neumann et al., 2019]] ) with the exposure to some pollen types estimated to double beyond present levels in Europe by 2041–2060 ( [[#Lake--2017|Lake et al., 2017]] ). The prospect of increases in summer thunderstorm events under climate change ( [[#Brooks--2013|Brooks, 2013]] ) may hold implications for changes in the occurrence of epidemic thunderstorm asthma ( [[#Bannister--2021|Bannister et al., 2021]] ; [[#Emmerson--2021|Emmerson et al., 2021]] ; [[#Price--2021|Price et al., 2021]] ). Similarly, projected alterations in hydroclimate under climate change may bear implications for increased exposure to mould allergens in some climates ( [[#D’Amato--2020|D’Amato et al., 2020]] ; [[#Paudel--2021|Paudel et al., 2021]] ).

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